Material for temperature compensation, and optical communication device

ABSTRACT

A temperature compensating member which comprises a polycrystalline body containing, as a main crystal, one of β-quartz solid solution and β-eucryptite solid solution, and it has a value less than 3.52 Å as an interplanar spacing of the crystal planes giving a main peak in X-ray diffraction measurement, and has a negative coefficient of thermal expansion.

CROSS REFERENCE TO RELATED APPLICATIONS

Applicants claim priority under 35 U.S.C. §119 of Japanese ApplicationNos. 192954/1999, 236201/1999, 135057/2000, and 180101/2000 filed Jul.7, 1999, Aug. 23, 1999, May 8, 2000, and Jun. 15, 2000, respectively.Applicants also claim priority under 35 U.S.C. §120 of PCT/JP00/04436filed Jul. 4, 2000. The international application under PCT article21(2) was not published in English.

TECHNICAL FIELD

This invention relates to a temperature compensating member having anegative coefficient of thermal expansion and an optical communicationdevice using the same.

BACKGROUND ART

With the advance of the optical communication technology, a networkusing optical fibers has been rapidly built up. In the network, awavelength multiplexing technique of collectively transmitting lightbeams having a plurality of different wavelengths has come into use, anda wavelength filter, a coupler, a waveguide, and the like have becomeimportant devices.

Some of the devices of the type described are changed in characteristicsdepending upon the temperature and may therefore cause troubles if usedin the outdoors. This requires a technique for keeping thecharacteristics of these devices fixed or unchanged regardless of atemperature change, i.e., a so-called temperature compensatingtechnique.

As a typical optical communication device which requires temperaturecompensation, there is a fiber Bragg grating (hereinbelow, referred toas FBG). The FBG is a device in which a portion varied in refractiveindex in a grid-like pattern, i.e., a so-called grating is formed withina core of an optical fiber, and has a characteristic of reflecting alight beam having a specific wavelength according to the relationshiprepresented by the following formula (1). Therefore, the device attractsattention as an important optical device in the optical communicationsystem using a wavelength division multiplex transmission technique inwhich optical signals different in wavelengths are multiplexed andtransmitted through a single optical fiber.

λ=2nΛ  (1)

Herein, λ represents a reflection wavelength, n, an effective refractiveindex of the core, and Λ, a grid interval of the portion varied inrefractive index in the grid-like pattern.

However, the above-mentioned FBG has a problem that the reflectionwavelength will be varied following the change in ambient temperature.The temperature dependency of the reflection wavelength is representedby the following formula (2) which is obtained by differentiating theformula (1) with the temperature T. $\begin{matrix}\begin{matrix}{{{\partial\lambda}/{\partial T}} = {2\left\{ {{\left( {{\partial n}/{\partial T}} \right)\Lambda} + {n\left( {{\partial\Lambda}/{\partial T}} \right)}} \right\}}} \\{= {2\Lambda \left\{ {\left( {{\partial n}/{\partial T}} \right) + {{n\left( {{\partial\Lambda}/{\partial T}} \right)}/\Lambda}} \right\}}}\end{matrix} & (2)\end{matrix}$

The second term of the right side of the formula (2), i.e., (∂Λ/∂T)/Λcorresponds to a coefficient of thermal expansion of the optical fiberand has a value approximately equal to 0.6×10⁻⁶/° C. On the other hand,the first term of the right side corresponds to the temperaturedependency of a refractive index of the core portion of the opticalfiber and has a value approximately equal to 7.5×10⁻⁶/° C. Thus, it willbe understood that the temperature dependency of the reflectionwavelength depends on both the variation in refractive index of the coreportion and the change in grid interval due to thermal expansion butmostly results from the temperature-dependent variation of therefractive index.

As means for avoiding the above-mentioned variation in reflectionwavelength, there is known a method in which the FBG is applied withtension depending upon the temperature change to thereby change the gridinterval so that a component resulting from the variation in refractiveindex is cancelled.

As a specific example of the above-mentioned method, proposal is made ofa method in which the FBG is fixed to a temperature compensating memberwhich comprises a combination of a material, such as an alloy or asilica glass, having a small coefficient of thermal expansion and ametal, such as aluminum, having a large coefficient of thermalexpansion. Specifically, as shown in FIG. 1, an Invar (trademark) bar 10having a small coefficient of thermal expansion has opposite endsprovided with Al brackets 11 a and 11 b having a relatively largecoefficient of thermal expansion attached thereto, respectively. Anoptical fiber 13 is fixed to these brackets 11 a and 11 b by the use ofclasps 12 a and 12 b so that the optical fiber is stretched under apredetermined tension. At this time, adjustment is made so that thegrating portion 13 a of the optical fiber 13 is located between the twoclasps 12 a and 12 b.

If the ambient temperature rises in the above-mentioned state, thebrackets 11 a and 11 b are expanded to reduce the distance between thetwo clasps 12 a and 12 b so that the tension applied to the gratingportion 13 a of the optical fiber 13 is decreased. On the other hand, asthe ambient temperature falls, the brackets 11 a and 11 b are contractedto increase the distance between the two clasps 12 a and 12 b so thatthe tension applied to the grating portion 13 a of the optical fiber 13is increased. Thus, by changing the tension applied to the FBG dependingupon the temperature change, it is possible to adjust the grid intervalof the grating portion. As a result, it is possible to cancel thetemperature dependency of the reflection center wavelength.

However, the above-mentioned temperature compensating device isdisadvantageous in that the structure is complicated and the handling isdifficult.

As a method for solving the above-mentioned disadvantages, JapaneseUnexamined Patent Publication No. 2000-503415 or Japanese UnexaminedPatent Publication No. 2000-503967 discloses a method shown in FIG. 2,in which a FBG 16 is, under a tension applied by a weight 15, fixed to aglass ceramic substrate 14 having a negative coefficient of thermalexpansion, by use of an adhesive 17, which substrate is obtained byheat-treating and crystallizing a raw glass material preliminarilyformed into a plate shape. The tension is controlled by expansion orcontraction of the glass ceramic substrate 14. In order to cancel thetemperature dependency of the reflection center wavelength, it isnecessary to apply a stress in a direction of contraction of the FBGwhen temperature rises and in a direction of expansion when temperaturefalls, as described above. As long as the substrate material has anegative coefficient of thermal expansion, such stress can be producedby a single component. The invention disclosed in the JapaneseUnexamined Patent Publication No. 2000-503415 or the Japanese UnexaminedPatent Publication No. 2000-503967 is achieved on the basis of thefunction and the effect mentioned above. In FIG. 2, 16 a represents agrating portion.

The method disclosed in the Japanese Unexamined Patent Publication No.2000-503415 or the Japanese Unexamined Patent Publication No.2000-503967 is advantageous in that the structure is simple and thehandling is easy because temperature compensation is achieved by asingle component. However, there is a problem that the glass ceramicmember used in the method is large in hysteresis of thermal expansion.The hysteresis of thermal expansion is a phenomenon in which, when amaterial expands or contracts following a temperature change, anexpanding behavior upon temperature elevation does not coincide withthat upon temperature drop.

In addition, the Japanese Unexamined Patent Publication No. 2000-503415or the Japanese Unexamined Patent Publication No. 2000-503967 disclosesa method for the purpose of diminishing the hysteresis of the glassceramic member, in which a heat-cycle treatment is carried out at atemperature between 400 and 800° C. to stabilize an internal structure.However, the hysteresis diminished by the method described above isunstable against a change in environment such as temperature or humidityand it is therefore difficult to maintain its initial value. Further,the above-mentioned heat treatment requires a complicated manufacturingprocess, resulting in a problem of a high cost.

Therefore, it is an object of the present invention to provide atemperature compensating member which is small in hysteresis of thermalexpansion, high in environmental stability, and capable of beingmanufactured at a low cost.

It is another object of the present invention to provide an opticalcommunication device using the above-described temperature compensatingmember.

DISCLOSURE OF THE INVENTION

In order to accomplish the above-mentioned objects, the presentinventors have conducted various experiments and, as a result, found outthat a temperature compensating member diminished in histeresis ofthermal expansion and excellent in environmental stability is obtainedby controlling the crystal structure of a polycrystalline body whichforms the temperature compensating member. This leads to a proposal ofthe present invention.

According to one aspect of the present invention, there is provided atemperature compensating member which comprises a polycrystalline bodycontaining, as a main crystal, one of β-quartz solid solution andβ-eucryptite solid solution, which has a value less than 3.52 Å as aninterplanar spacing of the crystal planes giving a main peak in X-raydiffraction measurement, and which has a negative coefficient of thermalexpansion.

The polycrystalline body may be a sintered powder body.

The above-mentioned temperature compensating member may have acoefficient of thermal expansion of (−25 to −120)×10⁻⁷/° C. within atemperature range between −40 and 100° C.

According to another aspect of the present invention, there is providedan optical communication device comprising the above-describedtemperature compensating member and an optical component having apositive coefficient of thermal expansion and fixed on one surface ofthe temperature compensating member.

The optical communication device may further comprise a reinforcingmember adhered to the other surface of the temperature compensatingmember by the use of an adhesive having a low elasticity.

The reinforcing member may be a columnar member having a through-hole,and the temperature compensating member may be placed in thethrough-hole of the reinforcing member.

The optical component may be fixed to the temperature compensatingmember by the use of an adhesive which comprises an organic polymer andhas a viscosity between 2500 and 100000 mPa·s at 25° C. prior to curingand a contracting rate of 5% or less upon curing.

It is noted here that the interplanar spacing means a distance betweenvarious crystal planes in the crystals forming the polycrystalline body.The present invention is concerned with the crystal plane giving themain peak in the X-ray diffraction.

BRIEF DESCRIPTION OF THE DRAWING

FIG. 1 is a front view showing a conventional device for preventingvariation in reflection wavelength of an FBG in response to atemperature change.

FIG. 2 is a perspective view showing a glass ceramic substrate having anegative coefficient of thermal expansion with the FBG fixed on itssurface.

FIG. 3 is a graph showing the correlation between an interplanar spacingof crystal planes of a polycrystalline body and a hysteresis.

FIG. 4 is a perspective view of an optical communication deviceaccording to a first embodiment of the present invention.

FIG. 5 is a perspective view of a characteristic part of an opticalcommunication device according to a second embodiment of the presentinvention.

FIG. 6 is a graph showing a thermal expansion curve of Sample No.2 as anexample.

FIG. 7 is a graph showing temperature dependency of a reflection centerwavelength of an FBG using a temperature compensating member whichcomprises Sample No. 2 as the example.

BEST MODE FOR EMBODYING THE INVENTION

A temperature compensating member according to an embodiment of thepresent invention comprises a polycrystalline body containing, as a maincrystal, one of β-quartz solid solution and β-eucryptite solid solution,has a value less than 3.52 Å as an interplanar spacing of the crystalplanes giving a main peak in X-ray diffraction measurement, and has anegative coefficient of thermal expansion.

FIG. 3 shows the correlation between the interplanar spacing of thecrystal planes of the polycrystalline body and a hysteresis. From thisfigure, it will be understood that, as the interplanar spacing becomessmaller, the hysteresis also becomes smaller. If the interplanar spacingis 3.52 Å or greater, the effect of diminishing the hysteresis as thetemperature compensating member is insufficient. In addition, thevariation rate of the hysteresis depending upon environmental factorssuch as temperature and humidity becomes large, resulting in adifficulty in obtaining a device having stable characteristics.

As the interplanar spacing of the crystal planes of the polycrystallinebody becomes smaller, the hysteresis also becomes smaller. However, ifthe interplanar spacing is excessively small, crystals of differentkinds are precipitated so that the coefficient of thermal expansion isshifted in a positive direction or the linearity of the thermalexpansion is deteriorated. Taking this into account, the interplanarspacing should be suitably selected depending upon applications orcharacteristics of the device. It is noted here that the interplanarspacing preferably has a value between 3.491 and 3.519 Å, morepreferably, between 3.495 and 3.512 Å.

As shown in IEICE General Conference C-3-46, 1997, even if a materialhaving a negative coefficient of thermal expansion is used as asubstrate of the type, the temperature dependency of a reflection centerwavelength may intensely be exhibited depending upon a temperature rangeso that a sufficient temperature compensating function cannot beobtained. This results from the poor linearity of thermal expansion ofthe substrate material.

Therefore, supposing a line (virtual line) connecting opposite ends of athermal expansion curve of the sample, finding a temperature at whichthe deviation of the measured curve from the virtual line is maximum,and calculating a value obtained by dividing a difference in samplelength between the virtual curve and the measured curve at theabove-mentioned temperature by an initial sample length before the test,it is desirable that the value thus calculated is restricted to 60 ppmor less. In this event, the linearity of the thermal expansion becomesexcellent so that a sufficient temperature compensating function can beobtained in any temperature range.

As a method of changing the interplanar spacing of the crystal planes ofthe polycrystalline body, there are various methods. For example, usemay be made of a method of adjusting a composition of thepolycrystalline body or a method of carrying out ion exchange afterproducing the polycrystalline body.

A preferable range of the composition suitable for diminishing theinterplanar spacing of the crystal planes of the polycrystalline bodyis, by weight %, 45-60% SiO₂, 20-45% Al₂O₃, 7-12% Li₂O, 0-4% TiO₂, and0-4% ZrO₂. By controlling the content of each of the ingredients to adesired value within the above-mentioned composition range, theinterplanar spacing can be adjusted to less than 3.52 Å. In addition tothe above-mentioned ingredients, it is possible to add other elements,such as MgO or P₂O₅, up to 10 weight %.

In case where the polycrystalline body is produced by re-heating andcrystallizing a glass obtained by melting a raw material followed bycooling and solidifying, adjustment of the interplanar spacing maysometimes be difficult in order to maintain a good meltability and agood formability of the glass. On the other hand, in case where thepolycrystalline body is produced by sintering powdery materials,adjustment is possible by the kinds and the ratios of the powderymaterials prior to sintering without being restricted by the meltabilityand the formability of the glass. Furthermore, not only a plate shapebut also any complicated shape can be easily formed at a low cost by amethod such as press forming, cast forming, or extrusion forming. Inview of the above, it is preferable to produce the polycrystalline bodyby sintering the powdery materials. As the powdery materials, use may bemade of amorphous glass powder, crystalline glass powder, partiallycrystallized glass powder, and glass powder prepared by a sol-gelmethod. Besides those mentioned above, sol or gel may be added.

Preferably, the temperature compensating member has the coefficient ofthermal expansion of (−25 to −120)×10⁻⁷/° C. (more preferably, (−50 to−90)×10⁻⁷/° C.) in a temperature range between −40 and 100° C.

Moreover, in case where the polycrystalline body is produced bysintering the powdery materials, it is possible to easily make a grooveor a through-hole at a predetermined position simultaneously with theforming. Thus, a great advantage is achieved in manufacturing of anoptical communication device. For instance, an optical fiber of an FBGis adhered and fixed to the temperature compensating member by the useof an adhesive (for example, glass frit or epoxy resin). If a groove ora through-hole is formed at a predetermined position of the temperaturecompensating member, assembling is easily automated upon carrying outthe adhering operation so that a production cost is lowered. The grooveor the through-hole is not restricted to one position but may be formedat a plurality of positions.

By selecting the diameter of the groove or the through-hole mentionedabove to be close to that of the device, it is possible to reduce theamount of the adhesive to be used and to achieve the fixation with athin adhesive layer. Such a thin adhesive layer decreases a stress dueto a difference in thermal expansion between the adhesive and each ofthe device and the temperature compensating member. Accordingly, it ispossible to perform the adhesion and the fixation throughout the overalllength of the groove or the through-hole and to prevent the device frombeing bent even if the temperature compensating member contracts fromthe length upon fixing.

Generally, upon fixing a fiber-shaped device such as an FBG to thetemperature compensating member, it is necessary to preliminarily applythe device with tension to prevent the device from being bent when thetemperature compensating member contracts from the length upon fixing.On the contrary, according to the present invention, it is unnecessaryto preliminarily apply the tension. Therefore, an optical device havinga temperature-compensating function can be produced in a simplerprocess. Especially, in case where a precise through-hole is formed inthe temperature compensating member and the device is inserted into thethrough-hole, the temperature compensating member serves as a componentfor positioning the device and as a connecting component when the devicehaving a temperature-compensating function is connected to an opticalfiber or another device.

Next referring to FIGS. 4 and 5, description will be made about opticalcommunication devices according to embodiments of the present invention.

The above-described temperature compensating member contains a crystalhaving anisotropy mainly in a behavior of thermal expansion, forexample, β-quartz solid solution. Therefore, one crystal axis of thecrystal having anisotropy in behavior of thermal expansion shows anextremely large negative coefficient of thermal expansion, that is, anegative coefficient of thermal expansion as large as −120×10⁻⁷/° C. atmaximum. The anisotropy in behavior of thermal expansion causes finegaps to be produced in a crystal grain boundary so that the mechanicalstrength tends to be decreased. Consequently, a problem may arise incase where a large stress is applied from the outside when the opticalcommunication device is assembled or the optical communication device isinstalled.

The optical communication device in FIG. 4 includes a temperaturecompensating member 18 of a plate shape having a negative coefficient ofthermal expansion. To one surface, for example, a bottom surface or aside surface of the temperature compensating member 18, a reinforcingmember 20 is adhered by the use of an adhesive 19 having a lowelasticity. To the other surface, for example, the top surface of thetemperature compensating member 18, an optical fiber 21 as an opticalcomponent having a positive coefficient of thermal expansion is adheredby the use of an adhesive 22 on both sides of a grating portion 21 a.

Since the above-mentioned optical communication device is high inmechanical strength, a problem will hardly arise even if a large stressis applied from the outside upon assembly or installation. Further,expansion and contraction of the temperature compensating member 18 dueto temperature changes are not easily prevented.

Herein, the adhesive 19 having a low elasticity is used in order thatthe expansion and contraction of the temperature compensating member 18due to temperature change are not easily prevented by the reinforcingmember 20 having a coefficient of thermal expansion greater than that ofthe temperature compensating member 18. Preferably, the adhesive 19contains a silicone resin because the adhesive 19 can be lowered inelasticity.

It is preferable that the reinforcing member 20 has a coefficient ofthermal expansion of 200×10⁻⁷/° C. or less in a temperature rangebetween −40 and 100° C. because the expansion and contraction of thetemperature compensating member 18 due to the temperature change are noteasily prevented. The reinforcing member 20 is not particularly limitedbut may be any material, such as metal, glass, and ceramic, as long asits mechanical strength is greater than that of the temperaturecompensating member 19. Especially, stainless steel, an Invar alloy, anda crystallized glass are preferable because they are excellent inchemical resistance so that the surface of the reinforcing member 20 isnot deteriorated and detachment hardly occurs at a boundary between theadhesive 19 and the reinforcing member 20. Moreover, the Invar alloy andthe crystallized glass are preferable because they are small incoefficient of thermal expansion to be therefore hardly preventexpansion and contraction of the temperature compensating member 18 dueto temperature change.

In the optical communication device in FIG. 5, a reinforcing member 20is formed into a columnar body having a through-hole 20 a. A temperaturecompensating member 18 with an optical fiber 21 fixed thereto isinserted into the through-hole 20 a and adhered to an inner wall surfaceof the through-hole 20 a by the use of an adhesive 19. Specifically, thereinforcing member 20 is formed into a cylindrical shape with ahorizontal center axis so as to surround the temperature compensatingmember 18 and the optical fiber 21. Herein, the columnar body means astructure such that an outer periphery of its cross section has asubstantially polygonal or circular shape.

In the above-mentioned optical communication device, not only themechanical strength is improved but also the reinforcing member 20serves to prevent and protect the optical fiber 21 from contamination orexternal force.

In addition, the reinforcing member 20 may be provided with a slitformed a substantially upper portion thereof to be in parallel to thethrough-hole 20 a. Alternatively, the substantially upper portion of thereinforcing member 20 may be partially cut away, and the cut-away partmay be used as a cover to open and close the through-hole 20 a. In thiscase, the optical fiber 21 need not be partially cut but can be fixed tothe temperature compensating member 18 while it is inserted in thethrough-hole 20 a. Therefore, workability is excellent.

An air-tight structure in which the both ends of the through-hole 20 aof the reinforcing member 20 are closed by covers (not shown) ispreferable in view of prevention of contamination or protection fromentry of water.

Furthermore, in each of FIGS. 4 and 5, it is preferable to preliminarilycoat the optical fiber 21 with a coating member (not shown) except agrating portion 21 a and an adhering portion to the temperaturecompensating member 18. This is because the optical fiber 21 will hardlybe damaged or broken by an edge portion of the temperature compensatingmember 18 or the reinforcing member 20 when the optical communicationdevice is assembled.

It is preferable that the adhesive 22 is made of an organic polymer,because the adhesion is possible in a short period of time and at a lowtemperature in comparison with an adhesive of glass or metal.

If the viscosity of the adhesive 22 prior to curing is 2500-100000 mPa·sat 25° C., the wettability of the adhesive 22 to the temperaturecompensating member 18 becomes adequate so that the adhesion is notreleased or loosened. If the viscosity of the adhesive 22 prior tocuring is less than 2500 mPa·s at 25° C., the wettability to thetemperature compensating member 18 becomes too high to keep the adhesivethroughout the entire periphery of the optical fiber 21, resulting in adecrease in adhesive strength between the optical fiber 21 and theadhesive 22. On the other hand, if the viscosity of the adhesive 22prior to curing is higher than 100000 mPa·s at 25° C., the wettabilityto the temperature compensating member 18 becomes poor, resulting in adecrease in adhesive strength between the adhesive 22 and thetemperature compensating member 18.

The adhesive 22 preferably has a contraction rate upon curing equal to5% or less. In this event, the tension applied to the grating portion 21a of the optical fiber 21 does not substantially increase upon curing.

It is noted here that the viscosity of the adhesive 22 at 25° C. priorto curing can be adjusted by selecting a kind, a molecular weight, and aconcentration of the polymer, a kind or an amount of a filler, an amountof a solvent, and so on. Further, the contraction rate of the adhesive22 upon curing can be reduced by an increase in degree of polymerizationof the polymer upon curing, an addition of the filler or an increase inamount thereof, and a decrease in amount of the solvent. Especially, anepoxy resin is preferable because its contraction rate upon curing issmall and has a great effect of decreasing a contraction rate of theadhesive upon curing.

Furthermore, it is preferable that the surface roughness (Ra) of aportion of the temperature compensating member 18 to which the adhesive22 is applied is 5 μm or less, because the wettability of the adhesive22 to the temperature compensating member 18 becomes good so that theadhesion is not released or loosened.

The wettability of the adhesive 22 to the temperature compensatingmember 18 can be evaluated by a contact angle. If the contact anglefalls within an angular range between 20° and 80°, the adhesion is notreleased or loosened so that the temperature compensating function isnot easily lost or deteriorated.

In addition, the adhesive 22 is preferably a UV-curing resin. TheUV-curing resin is easily adhered in a short period of time and at a lowtemperature. If UV curing is followed by heat treatment at a temperatureat which the characteristics of the optical component are notdeteriorated, the adhesive strength increases although a curing periodbecomes slightly longer.

The above-described optical communication device has a stabletemperature compensating characteristic and is high in mechanicalstrength, because of use of the temperature compensating member which issmall in hysteresis of thermal expansion and high in stability of thehysteresis against environmental changes.

In the foregoing, the example using the optical fiber as an opticalcomponent is shown. However, this invention is similarly applicable tothe cases where optical components of other types are used.

Hereinbelow, description will be made in detail about the temperaturecompensating member in conjunction with various examples and acomparative example.

Table 1 shows the examples of the present invention (Samples Nos. 1-6)and the comparative example (Sample No. 7).

TABLE 1 Comparative Example Example Sample No. 1 2 3 4 5 6 7 SiO₂ 56.755.2 50.7 57.1 58.0 46.2 44.5 Al₂O₃ 31.6 33.0 36.8 30.4 31.0 40.9 43.0Li₂O 8.6 9.3 11.1 8.1 7.7 9.1 12.5 TiO₂ 1.0 0.8 0.4 1.1 0.8 1.9 — ZrO₂1.2 1.0 0.5 1.6 1.4 1.9 — MgO 0.2 0.2 0.1 0.3 0.3 — — P₂O₅ 0.7 0.5 0.41.4 0.8 — — Kind of crystal β-Q β-Q β-Q β-Q β-Q β-E β-E s. s. s. s. s.s. s. s. s. s. s. s. s. s. Interplanar spacing (Å) 3.498 3.501 3.5193.496 3.493 3.515 3.534 Coefficient of thermal −57 −78 −95 −34 −26 −64−98 expansion (×10⁻⁷/° C.) Hysteresis (ppm) Initial Value 18 23 57 8 755 78 After high-temperature 20 26 69 8 7 68 150 high-humidity

In Table 1, each of Samples Nos. 1-5 and No. 7 was prepared in thefollowing manner. At first, raw materials were blended so that thepolycrystalline body after sintering would have the composition (weight%) in the table. Thereafter, the blended batch was put in a mold andpress-formed under the pressure of 20 MPa to produce a molded body (acompact body) having a rectangular-section columnar shape of a width of4 mm, a thickness of 3 mm, and a length of 40 mm. Then, the molded bodywas sintered at 1350° C. in air for 15 hours and then cooled to the roomtemperature to obtain a polycrystalline body of β-quartz solid solution.

As regards Sample No. 6, the raw materials were blended so that thepolycrystalline body after crystallization would have the composition(weight %) in the table. Thereafter, the blended batch was melted at1500° C. for 7 hours, and rapidly cooled to produce a glass. Then, theglass was heated at 1350° C. for 15 hours to be crystallized. Thus, thepolycrystalline body in which β-eucryptite solid solution wasprecipitated was obtained.

The raw materials of the polycrystalline body can be suitably selectedfrom various minerals and compounds. In the table, β-Qs.s. representsβ-quartz solid solution while β-Es.s. represents β-eucryptite solidsolution.

As apparent from Table 1, each of Samples Nos. 1-6 comprised β-quartzsolid solution or β-eucryptite solid solution, had a negativecoefficient of thermal expansion within a range of (−26 to −95)×10⁻⁷/°C., and had a small interplanar spacing less than 3.52 Å. Therefore, aninitial hysteresis is small and the variation in hysteresis after ahigh-temperature high-humidity test is also small. Thus, these samplesare suitable as the temperature compensating member. In addition, eachsample had a linearity of thermal expansion of 60 ppm or less.

On the other hand, Sample No. 7 had an interplanar spacing as large as3.534 Å so that the initial hysteresis and the change in hysteresisafter the high-temperature high-humidity test were great. In addition,the linearity of thermal expansion was greater than 60 ppm. Thus, thissample was inappropriate as the temperature compensating member.

FIG. 6 is a graph showing a thermal expansion curve of Sample No. 2.FIG. 7 is a graph showing temperature dependency of the reflectioncenter wavelength of the FBG using the temperature compensating memberformed by Sample No. 2. From FIG. 6, it is understood that Sample No. 2exhibits a good linearity of thermal expansion. From FIG. 7, it isunderstood that the temperature dependency of the reflection centerwavelength of the FBG having temperature compensation is very small incomparison with the case of no temperature compensation and is stable inany temperature range.

An optical communication device comprising Sample No. 2 (4×40×2 mm) anda stainless steel plate (4×40×1 mm) adhered to the bottom surfacethereof by an adhesive of a silicone resin had a breaking load of 9 kgfcorresponding to a high mechanical strength, and was small intemperature dependency of the reflection center wavelength.

On the other hand, another optical communication device comprisingSample No. 2 which was not reinforced by a reinforcing member had abreaking load of 1.5 kgf corresponding to a low mechanical strength.

Another optical communication device was produced by adhering an FBG tothe top surface of the temperature compensating member comprising SampleNo. 2 by the use of an adhesive of an epoxy resin having a viscosity of4000 mPa·s at 25° C. and a contracting rate of 0.2% upon curing. In thisdevice, the characteristics of the FBG were not degraded because theadhesion could be carried out at a low temperature. Further, since thewettability of the adhesive to the temperature compensating member wasexcellent, the adhesion was not released or loosened. Furthermore, sincethe contract rate upon curing was low, the temperature compensatingfunction was neither lost nor deteriorated by an increase in tensionapplied to the optical fiber.

The kinds of crystals in Table 1 and the interplanar spacing of thecrystal planes giving a main peak were determined by the X-raydiffraction. The coefficient of thermal expansion and the hysteresiswere measured by a dilatometer. The coefficient of thermal expansion wasmeasured within a temperature range between −40 and 100° C. Thehysteresis was obtained by repeatedly heating and cooling each samplewithin a temperature range between −40 and 100° C. at a rate of 1°C./min., measuring a difference between the lengths of the sample at 30°C. during heating and during cooling, and dividing the difference by theinitial sample length before the test. The hysteresis after ahigh-temperature high-humidity test was a value obtained after leavingin an environment of 70° C. and 85% RH for 500 hours. The breaking loadwas measured by a method according to JIS R 1601.

The viscosity of the adhesive was measured by the use of a viscometer ofa HB type (manufactured by Brookfield Corp.) at 25° C. and at a shearrate of 4S⁻¹. The contracting rate upon curing was measured by aspecific gravity cup method.

Each of the above-described temperature compensating members is small inhysteresis of thermal expansion, has a high stability of hysteresisagainst environmental changes, and can be produced at a low cost.Therefore, use is suitably made as the temperature compensating memberin an optical communication device such as the FBG, a coupler, and awaveguide.

INDUSTRIAL APPLICABILITY

The temperature compensating member according to the present inventionis suitable as the temperature compensating member in the opticalcommunication device such as the FBG, the coupler, and the waveguide.

What is claimed is:
 1. A temperature compensating member which comprisesa polycrystalline body containing, as a main crystal, one of β-quartzsolid solution and β-eucryptite solid solution, which has a value lessthan 3.52 Å as an interplanar spacing of the crystal planes giving amain peak in X-ray diffraction measurement, and which has a negativecoefficient of thermal expansion.
 2. A temperature compensating memberas claimed in claim 1, wherein said polycrystalline body is a sinteredpowder body.
 3. A temperature compensating member as claimed in claim 1,wherein said member has a coefficient of thermal expansion of (−25 to−120)×10⁻⁷/° C. within a temperature range between −40 and 100° C.
 4. Anoptical communication device comprising a temperature compensatingmember claimed in claim 1 and an optical component having a positivecoefficient of thermal expansion and fixed on one surface of saidtemperature compensating member.
 5. An optical communication device asclaimed in claim 4, further comprising a reinforcing member adhered tothe other surface of said temperature compensating member by the use ofan adhesive having a low elasticity.
 6. An optical communication deviceas claimed in claim 5, wherein said reinforcing member is a columnarbody having a through-hole, said temperature compensating member beingplaced in the through-hole of said reinforcing member.
 7. An opticalcommunication device as claimed in claim 4, wherein said opticalcomponent is fixed to said temperature compensating member by the use ofan adhesive, said adhesive comprising an organic polymer and having aviscosity between 2500 and 100000 mPa·s at 25° C. prior to curing and acontracting rate of 5% or less upon curing.